Oscillators usually are of two broad general types namely, the non-sinusoidal wave type and the sinusoidal wave type. The non-sinusoidal type, typically a squarewave or a sawtooth wave oscillator, has frequently been used when it was desired to provide the ability to vary the frequency of oscillation within predetermined limits. The frequency variation is achieved by varying the ramp currents in the circuit which in turn changes the duration of the ramp signal and therefore the frequency of the oscillator. However, problems have always existed in such oscillators because of the generation of harmonic frequencies and noise. Because of the necessity of changing the slope of the ramp signals frequently and abruptly, there exists the tendency to not only generate noise but generate the many harmonics of the basic frequency desired. Thus the application of non-sinusoidal oscillators has been limited to areas where the harmonic frequencies and noise could be endured or otherwise extensive steps must be taken to suppress and filter out the unwanted signals. Still the necessity to use the non-sinusoidal oscillator has existed because of the need to vary the frequency of oscillation.
In contrast, sinusoidal type oscillators have not been plagued with the generation of noise and harmonic frequencies in the manner previously described with respect to sawtooth oscillators because of the basic usage of an internal waveform approaching the sinusoidal waveform. Thus from this standpoint the sinusoidal oscillator has been applied whenever possible to those applications requiring quiet and clean signal forms. However, the frequency of oscillation of a sinusoidal oscillator can vary with the values of the components within the circuit. In other words, the frequency of oscillation is directly dependent on the component values and therefore the accuracy with which one can make a sinusoidal oscillator oscillate at a predetermined frequency depends upon the accuracy with which one can manufacture the components that go into the circuit.
To provide a variable frequency sinusoidal oscillator the usual method has been to utilize a Varicap in the feedback circuit to vary the phase angle and thus shift the resonant frequency of the oscillator. Difficulties arise in that Varicaps require a high dc voltage and small oscillatory voltages for operation and since the overall frequency of oscillation depends upon the reactive values of all the components, the frequency of oscillation is further dependent upon the accuracy and tolerances to which these components were held. Thus there remains to be provided an effective variable frequency oscillator of the sinusoidal type and it is to this purpose that the subject invention is directed.